Consumption and inflation were listed, but I think the biggest reason is social constructs. It's just assumed that you should work full time or parent full time. A lot of people's stance on early retirement is what else would I do with my time? People who don't work full time and instead do leisurely activities like gaming, reading, watching tv are seen as lazy and unproductive.
Plus we invent new stuff to work on, new industries like entertainment, computers etc. ~300 years ago how many people were able to abandon the farm and go to school and/or become engineers/inventors/artists/whatever? Not many cus it took a lot more work to do basic things like produce food, clothes, shelter. Now those things are relatively cheap as a sustinence level, so new industries arise like the internet. Look at how many people facebook and google employ and what actual productive things are they doing? Their business is data, it's not really tangible but it has monetary value obviously. We make up new things to work on.